1. Field of the Invention
The present invention relates to a digitizer, and more particularly to a digitizer with a charge pump circuit that has no PN current mismatch.
2. Description of the Prior Art
In read channels of many optical storage systems, such as CD and DVD systems, a data slice circuit for slicing analog signals into digital ones is needed for following process. The data slice circuit, which is called as the digitizer or the data detector briefly, needs to slice analog data accurately at a correct slice level. Otherwise, the error rate of digital data increases, and the performance of the read channel reduces.
According to the specification of these systems, the digital sum value (DSV) of digital signals in the read channels should be minimized to 0. And it is referred to the symmetry of signals. Hence the correct slice level of the digitizer is the slice level at which the digitizer may slice analog data accurately and keep the symmetry of digital signals. However, the dc level of the input analog signals drifts easily and components in the circuit may provide offset in various level. To avoid errors due to the inaccurate slicing, various methods for adjusting the slice level in the read channels are adopted in the prior art.
Please refer to FIG. 1. FIG. 1 is a block diagram of a conventional digitizer 100. The conventional digitizer 100 includes a comparator 110, an inverter 120, a charge pump 130, a pump capacitor 140, and a low pass filter 150. The comparator 110 receives differential analog signals at inputs INP and INN. The output signals of the comparator 110 are sent to other components for following process. Besides, the output signals of the comparator 110 and the inverse output signals of the inverter 120 are sent to the charge pump 130 as the Up and Dn signals respectively. The inverter 120 simply inverses the output signals Up of the comparator 110 to generate the inverse output signals Dn. When the output signal of the comparator 110 is high, the charge pump 130 charges the pump capacitor 140; and when the output signal of the comparator 110 is low, the charge pump 130 discharges the capacitor 140. The output of the charge pump 130 is further coupled to the low pass filter (LPF) 150, so that the voltage at the capacitor 140 is filtered before being fed back to the comparator 110. Furthermore, the comparator 110 receives a reference voltage, Vref, and the filtered output of the LPF 150 to adjust the slice level.
In the conventional digitizer 100, the charge pump 130 of a common structure is adapted to fulfill the feedback loop of slice level calibration. Please refer to FIG. 2. FIG. 2 is a schematic of the charge pump 130 and the pump capacitor 140. As shown in FIG. 2, the charge pump 130 includes a charge current source 131 and a discharge current source 132, of which the current values are Iup1 and Idn1 respectively. The charge current source 131 and the discharge current source 132 are coupled to switches 133 and 134 respectively. The switch 133 is selectively coupled to a loading capacitor 135 or the pump capacitor 140 according to the Up signal received from the comparator 110 (not shown in FIG. 2). Similarly, the switch 134 is selectively coupled to a loading capacitor 135 or the pump capacitor 140 according to the Dn signal, which is the inverse of the Up signal generated by the inverter 120 (not shown in FIG. 2). Since the Up signal and the Dn signal are inverse to each other, the loading capacitor 135 is coupled to either the current sources 131 or 132 at a time, meanwhile the pump capacitor 140 is coupled to the other current sources 132 or 131. In this manner, the charge pump 130 and the pump capacitor 140 together fulfill the loop of reflection of the sum of data level.
However, the charge current source 131 and the charge current source 132 are not physically identical, and the switching speeds of the switches 133 and 134 are different. For example, the two current sources may be composed of PMOS and NMOS independently and hence have different characteristics called PN current mismatches. The switches 133 and 134 are not identical either. Therefore, in consideration of the comparator 110 to output signals at high level and low level equally, the charge current and the discharge current of the pump capacitor 140 may not be equal. Consequently, the voltage fed back to the comparator 110 is not as stable as expected.
There are ways to calibrate the mismatch between the charge current source and the discharge current source of the charge pump adopted in the digitizer. Please refer to FIG. 3. FIG. 3 is a block diagram of a calibration circuit 300 of the prior art. The calibration circuit 300 includes two current sources 331 and 332, which are duplicates of the charge current circuit and the discharge current circuit adapted in the charge pump respectively. A resister 340 is coupled to the joint of the two current sources 331 and 332. Hence the current biasing the resistor 340 is:Im1=Iup1−Idn1;  eq. (1)and the voltage at the resistor R is:Vm=Im1·R  eq. (2)
The resistor 340 is further coupled to an analog to digital converter (ADC) 350. The ADC 350 receives the voltage Vm and outputs a corresponding digital signal to a micro-processor 360. The micro-processor 360 generates a control signal Sc in accordance with the input signals from the ADC 350 to adjust the current value of the current source 331 for reducing the current mismatch Im1. The charge current source of the charge pump then can be adjusted according to the control signal Sc as well or according to the adjusted current source 331 of the calibration circuit 300. In this manner, the mismatch between the charge current source and the discharge current source can be decreased.
Described above is the conventional calibration mechanism adopted in charge pumps of digitizers. However, there is a serious drawback in this conventional calibration method. The charge current source 131 and the discharge current source 132 do not function simultaneously in the charge pump circuit 130, and the two current sources 331 and 332 are turned on at the same time while calibrating the mismatch in the calibration circuit 300. That means, the current values of the two current sources 331 and 332 can never be identical to the charge current source 131 and the discharge current source 132. Hence the calibration circuit 300 of the prior art is not able to cancel the current mismatch between current sources of the charge pump exactly.